Minimum Price?

The price P of a certain computer system decreases immediately after its introduction and then increases. If the price P is estimated by the formula , where t is the time in months from its introduction, find the time until the minimum price is reached.

The price P of a certain computer system decreases immediately after its introduction and then increases. If the price P is estimated by the formula , where t is the time in months from its introduction, find the time until the minimum price is reached.